6.3 Load Sharing, Load Shedding & Spinning Reserve
Key Takeaways
- Online generators share real power (kW) and reactive power (kVAr); healthy sharing keeps individual sets within ratings and frequency/voltage stable
- Spinning reserve is the unused online generating capacity available for sudden extra demand without waiting for a cold start
- For DP, spinning reserve must cover thruster demand spikes and the post-generator-failure load shift assumed in the operating philosophy
- Load shedding and thruster power limiting protect the bus by reducing non-essential and/or thruster load before underfrequency blackout
- Exam scenarios with thruster demand spikes test whether you prioritise plant survival (limit/shed, start standby) over unconstrained thrust that collapses generation
From “enough generators” to “enough reserve right now”
Having diesel-electric architecture and a working PMS is not enough if the online plant cannot absorb the next load step. This section links three ideas that dominate DP power questions:
- Load sharing — how parallel gensets divide the work.
- Spinning reserve — spare online capacity for sudden demand or a set trip.
- Load shedding / thruster limiting — deliberate load reduction to save the bus.
If you confuse “installed capacity in the engine room” with “capacity actually on the board with headroom,” you will mis-answer both MCQs and simulator power failures.
Load sharing: kW and kVAr
When two or more generators operate in parallel on a bus, they must share load:
| Quantity | Name | Roughly related to | Unhealthy if… |
|---|---|---|---|
| kW | Real / active power | Engine fuel / torque; does useful work (thrusters, pumps) | One set overloaded while another is lightly loaded |
| kVAr | Reactive power | AVR / excitation; supports voltage and magnetising currents | One set absorbs or supplies excessive vars; voltage unstable |
kW sharing is largely a governor / load-share control problem. kVAr sharing is largely an AVR / reactive-load-share problem. PMS and generator control systems keep both balanced. As DPO you read percentage load and alarms: a set at 95% kW while its twin sits at 40% is a sharing or control fault, not “normal diversity.”
Poor kW sharing risks:
- thermal overload of the heavy set,
- reverse power trip of the light set if it motoring-absorbs power,
- reduced total reliability because protection may trip the overloaded machine.
Poor kVAr sharing risks voltage problems and AVR stress even when kW looks acceptable. Exam stems may say “generators not sharing load equally” — your association should be investigate / reduce risk / call engineer, not ignore because total kW still covers average demand.
Spinning reserve: DP definition
Spinning reserve is the margin between present online load and present online generating capacity. It is power you can call on immediately (subject to engine/generator dynamic limits) because the machines are already running and synchronised. It is not:
- a cold generator still locked out in the workshop,
- nameplate total of all gensets including those offline,
- UPS battery ampere-hours for computers,
- the thruster’s maximum continuous rating alone.
| Term | Meaning for DP exams |
|---|---|
| Online capacity | Sum of ratings of gensets actually on the bus |
| Online load | Present kW (and watch kVAr) drawn from that capacity |
| Spinning reserve | Online capacity − online load (plus policy about usable dynamic margin) |
| Standby generator | Ready to start, but not yet spinning reserve until online |
Why DP cares: thruster demand is spiky. Gusts, current changes, thruster bias, or a heading change can raise thruster kW in seconds. A plant sitting at 90% average load with no reserve may look fine until a spike drives underfrequency and cascade trips. Standing orders and ASOG often specify minimum spinning reserve or minimum online gensets during critical activities precisely for this reason.
Also consider post-failure reserve. If you lose one generator, load shifts to remaining sets. If those remaining sets were already near full load, they may overload and trip — cascade blackout. Adequate spinning reserve is therefore both:
- headroom for weather/thruster spikes, and
- headroom so the worst remaining plant after a single failure can still supply essential DP load (aligned with FMEA/operational philosophy).
Load shedding: priorities that protect DP essential load
Load shedding is the controlled removal (or forced reduction) of electrical load when generation cannot support demand. On DP vessels it appears as:
- automatic preferential trip stages of non-essential consumers,
- thruster power limitation from PMS to DP/drives,
- manual engineer/DPO actions to stop heavy industrial consumers.
Conceptual priority for exams (vessel lists vary—learn the principle):
| Priority band | Examples | Intent |
|---|---|---|
| Shed early | Non-essential hotel, non-critical deck consumers | Free kW without killing station-keeping |
| Limit next | Thruster demand capping via PMS interface | Keep bus alive; accept larger footprint temporarily |
| Protect longest | DP control computers, critical sensors (often UPS), essential thrusters within remaining capability | Preserve ability to recover position control |
| Never “fix first” in good practice | Randomly tripping DP process stations or all thruster breakers | That creates the blackout/drift you are trying to avoid |
Load shedding is not a punishment; it is plant CPR. A short period of power-limited thrusters is usually preferable to a blackout recovery measured in minutes under a platform.
Underfrequency: the bus’s distress signal
When load exceeds what engines can deliver, frequency falls (underfrequency). Protection may trip generators or large consumers; thrusters may cut back. Underfrequency is a late symptom that reserve and load control were insufficient for the event. PMS design tries to act before deep underfrequency by starting sets, limiting thrusters, and preferential tripping. If you see underfrequency alarms during DP:
- Assume plant distress — not a sensor curiosity.
- Expect thruster limiting / shed actions.
- Reduce operational risk (stop critical external ops per ASOG).
- Confirm standby start and restore reserve with engineering.
- Do not force thrusters to 100% against power limits to “save position” if that collapses the remaining generators.
Exam scenarios: thruster demand spike
Scenario A — Gust with good reserve. Four gensets online at 55% load. Thrusters spike to meet a gust. Frequency dips slightly; spinning reserve absorbs the step; position holds. Lesson: reserve turns weather into a non-event.
Scenario B — Spike with thin reserve. Two gensets at 88% load, auto-stop inhibited wrongly left with only two sets in rising weather. Thruster demand jumps; underfrequency; preferential trip of hotel; thruster limit engages; footprint grows. If the DPO closes bus-tie purely “to add power” without understanding fault risk and without extra generation, they may create a worse common-mode failure path. Correct path: start more generation, accept temporary thruster limit, reduce work status if required.
Scenario C — Generator trip during high thruster load. One of three online gensets trips. PMS redistributes kW, applies thruster limit, starts standby. If remaining two were already at 90%, only thruster limiting prevents cascade. Lesson: spinning reserve must survive N-1 generator loss during critical DP, not only average weather.
Scenario D — Wrong response menu. Candidate chooses “open all thruster breakers” or “trip remaining generators to reset.” Both destroy station-keeping power. Correct PMS-centred response preserves remaining generation and reduces demand.
| Observation | Better interpretation | Poor leap |
|---|---|---|
| Thrusters power-limited after generator trip | PMS protecting bus | “DP software deleted thrusters forever” |
| Frequency low, loads shedding | Plant overload / insufficient reserve | “Only a gyro problem” |
| One genset 95%, twin 40% | Load share fault | “Normal because thrusters prefer one engine” |
| Standby not yet on board | Not spinning reserve yet | “Nameplate capacity equals online reserve” |
Operator checklist before critical DP
- Confirm number of gensets online matches activity/ASOG.
- Confirm spinning reserve adequate for weather forecast and thruster bias/load.
- Confirm standby is ready (fuel, start air, breakers, no inhibit that blocks auto-start).
- Know whether thruster load limitation is active/available.
- Know major preferential trip groups that might drop (communicate with ECR).
- After any plant change, re-check load percentages and residual capability mentally against consequence analysis / footprint.
Linking forward
Open vs closed bus-tie changes how reserve is shared versus how faults propagate (next chapter). Blackout recovery is what happens if sharing, reserve, and shedding all lose the race. Drive-off vs drift-off distinguishes unwanted thruster force from insufficient thruster force—power starvation is a classic drift-off path.
Bottom line: balanced kW/kVAr sharing, deliberate spinning reserve, and disciplined load shedding / thruster limiting keep the electrical plant from collapsing when thrusters spike or a generator dies. On the exam, protect the bus first; unconstrained thrust that blackouts the vessel protects nothing.
What is the purpose of running generators with adequate online spinning reserve during DP critical operations?
When generator load approaches its limit, PMS load shedding typically means:
On a DP vessel, which pair of quantities do parallel generators share under healthy load-share control?
A thruster demand spike drives bus frequency down while two gensets are already near full load. Which response best matches good DP power-management practice?